Astrophysics

Abstract: I discuss simulations of the coalescence of black hole neutron star binary
systems with black hole masses between 14 and 20 \msun. The calculations use a
three-dimensional smoothed particle hydrodynamics code, a
temperature-dependent, nuclear equation of state and a multi-flavor neutrino
scheme. General relativistic effects are mimicked using the \Pacz-Wiita
pseudo-potential and gravitational radiation reaction forces. Opposite to
previous, purely Newtonian calculations, in none of the explored cases episodic
mass transfer occurs. The neutron star is always completely disrupted after
most of its mass has been transferred directly into the hole. For black hole
masses between 14 and 16 \Msun an accretion disk forms, large parts of it,
however, are inside the last stable orbit and therefore falling with large
radial velocities into the hole. These disks are (opposite to the neutron star
merger case) thin and -apart from a spiral shock- essentially cold. For higher
mass black holes ($M_{\rm BH} \ge 18$ \msun) almost the complete neutron star
disappears in the hole without forming an accretion disk. In these cases the
surviving material is spun up by tidal torques and ejected as a half-ring of
neutron-rich matter. None of the investigated systems is a promising GRB
central engine. We find between 0.01 and 0.2 \Msun of the neutron star to be
dynamically ejected. Like in a type Ia supernova, the radioactive decay of this
material will power a light curve with a peak luminosity of a few times
$10^{44}$ erg/s. The maximum will be reached about three days after the
coalescence and will be mainly visible in the optical/near infrared band. The
coalescence itself may produce a precursor pulse with a thermal spectrum of
$\sim 10$ ms duration.